Redundancy in process control system

ABSTRACT

The invention relates to a method and an apparatus for arranging redundancy in a process control system comprising at least one client device, at least one data source and at least one server device providing data transfer between the client device and the data source. The method comprises arranging, for the client device, at least two parallel logical connections for transferring substantially the same data between at least one data source and the client device. At the client device, property information on data units transferred via different logical connections is checked, the client device being provided with predetermined order criteria for the property information. At the client device, property information on parallel data units transferred via different logical connections is compared. Data units delivered via the logical connection having, according to the predetermined order criteria, the best property information are selected for the use of one or more applications processing the data units.

This is a Continuation of Application No. PCT/FI03/00176 filed Mar. 7,2003. The entire disclosure of the prior application is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to arranging redundancy in a process controlsystem.

In an industrial process, a highly automatized system is used forensuring that the process operates in a correct manner. An industrialprocess, such as a paper making process or a power station process, isvery extensive and complex, including many variables. An informationsystem used for controlling an industrial process is responsible forvarious tasks relating to collecting, distributing, storing andpresenting process properties as well as to process control. A processcontrol system typically comprises a large number of work stations thatoperate independently or controlled by an operator. Such work stationscarry out different process control related tasks, such as processing ofmeasurement data and storage of historical data, according to theapplications to be executed in the work stations.

Conventionally, process control systems have employed proprietaryhardware and software solutions, in which case devices from differentmanufacturers are unable to communicate with each other. In such a case,it has been necessary to determine proprietary interfaces forapplications in order to access data of different devices. In theprocess control industry, several standards have thus been developed todetermine open communication interfaces to be used between the differentdevices of a process control system. An example of such an open standardis OPC (Object Linking and Embedding for Process Control). The OPCprovides a set of interfaces, properties and methods based onActiveX/COM (Component Object Model) technologies to be used for processcontrol applications. A bus solution supporting the OPC standardcomprises OPC clients and OPC servers. The OPC servers may communicatewith proprietary devices and transfer data to different OPC clients thatforward data to applications utilizing the data. The OPC enables acommon interface to be provided for the applications so as to enableaccess to the data of different process control devices.

The functionality of OPC servers serving up to dozens of OPC clientsthus plays a crucial role in the information system. In order to ensurethe functionality of a system, redundancy of OPC servers may thus bearranged by doubling, meaning that in addition to a primary OPC server,the system is also provided with a secondary OPC server. Such aredundancy arrangement has been disclosed in WO 0 023 857, the solutionaccording to the publication comprising switching over to using thesecondary OPC server when the primary OPC server is no longer available.The secondary server may start carrying out the tasks of the primaryserver without the client devices noticing the change (the logicalconnection provided for the client device remains unchanged although theserver device providing the connection changes). The solution disclosedin WO 0 023 857 does not, however, take any other error situations butserver or connection failure into account, thus providing limitedredundancy. Even if a server were working correctly, the connection e.g.between the server and a field or another device connected thereto maybe completely lost or otherwise subjected to interference.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide an improved redundancy solutionfor process control systems. The objects of the invention are achievedby a method, a process control system, a process control system deviceand a computer program which are characterized by what has beendisclosed in the independent claims. Preferred embodiments of theinvention are disclosed in the dependent claims.

The idea underlying the invention is that a client device is providedwith at least two parallel logical connections for transferringsubstantially the same data between at least one data source and theclient device. Property information on parallel data units transferredvia different logical connection is checked at the client device andcompared. The client device is provided with predetermined ordercriteria according to which the parallel data units can be arranged inorder of superiority. Parallel data units refer to data units whosepayload to be used by an application is exactly or substantially thesame. Data units delivered via the logical connection having, accordingto the predetermined order criteria, the best property information areselected for the use of one or more applications processing the dataunits. A logical connection refers to an arrangement formed by at leasttwo data transfer entities for transferring data in a process controlsystem; parallel logical connections are not necessarily arranged viaphysically separate data transfer devices. Data transfer via a logicalconnection may be either circuit- or packet-switched.

An advantage of the arrangement of the invention is that it is alwayspossible to select the best ones from among parallel data unitstransferred via parallel connections. This enables redundancy to bearranged so as to also takes into account problems in data transfer thatimpair the quality of the data units transferred. Redundancy based oncomparing the property information on parallel data units provideshigh-speed action as far as the application using the data units isconcerned also when some of the connections fails, i.e. a delay causedby activating a new, substitutive connection (e.g. via a secondaryserver) is avoided.

According to a preferred embodiment of the invention, the propertyinformation to be compared consists of quality information associated bya server device supporting the OPC standard to OPC items, and thepredetermined order criteria are based on arranging quality levelsdetermined in different OPC specifications in order of superiority. Thisenables quality data fields determined already in the OPC specificationsto be utilized in the redundancy of OPC connections.

According to yet another preferred embodiment of the invention, theproperty information to be compared consists of time stamps of the dataunits, and at the client device, the data units that have the mostrecent time stamps are selected from among the parallel data units ofthe logical connections. This enables the most recent possibleinformation to be provided for the application. This embodiment may alsocomplement the comparison of quality information preferably when,according to the quality information, parallel data units are equal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail in connection with thepreferred embodiments and with reference to the accompanying drawings,in which

FIG. 1 is a block diagram generally showing a process control system;

FIG. 2 is a block diagram showing the operational structure of a serverof a process control system according to a preferred embodiment of theinvention;

FIG. 3 is a block diagram showing a method of a preferred embodiment ofthe invention;

FIG. 4 is a block diagram showing a redundancy configuration accordingto a first embodiment;

FIG. 5 is a block diagram showing a redundancy configuration accordingto a second embodiment;

FIG. 6 is a block diagram showing a redundancy configuration accordingto a third embodiment; and

FIG. 7 is a block diagram showing a redundancy configuration accordingto a fourth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the example in FIG. 1, a process control system comprisesOPC clients OPCC, OPC servers OPCS, and devices DEV connected to the OPCservers. According to the applications to be executed therein, workstations WS comprising an OPC client OPCC may carry out various tasks:They may operate e.g. as engineering workstations, backup workstations,process control servers, diagnostics servers, operator servers or asalarm servers. Database workstations transferring data from a processand control information to the process are typically called processcontrol servers.

An OPC server OPCS may be connected to a large variety of devices DEV,which usually require a proprietary interface. The devices DEV may bee.g. field devices, I/O units connected to field devices, databases,system bus devices, such as process control servers or OPC servers ofother systems. An OPC server OPCS provides an interface for datasources, OPC items, and controls them. An OPC server OPCS is any serverdevice providing one or more OPC interfaces, including at least an OPCdata transfer interface (OPC DataAccess) for reading and writing data,an OPC alarm and event interface (OPC Alarm&Event) for indicatingspecified alarms and events to the OPC client OPCC, and an OPChistorical data interface (OPC HistoricalData) for reading, processingand editing historical data. It is to be noted that in addition to theabove-mentioned functions, also other OPC functionalities may bedeveloped. OPC clients OPCC and OPC servers OPCS communicate e.g. overthe Ethernet network.

Devices providing functions (OPCC, OPCS) according to a preferredembodiment comprise a memory, a user interface (typically at least adisplay, a keyboard and a mouse), means for transferring data (typicallya network interface card and software controlling data transfer) and acentral processing unit CPU comprising one or more processors. Aninformation system may, however, also comprise devices to be controlledvia a network, comprising no user interface. The memory comprises anon-volatile part for storing program code controlling the CPU and otherdata to be stored, and a volatile part for storing data temporarily.

FIG. 2 illustrates the operational structure of a device (WS) comprisingan OPC client functionality of a process control system according to apreferred embodiment. The device comprises one or more applications APPand an OPC client functionality OPCC. The OPC client functionality OPCCcomprises an OPC redundancy functionality (Redundancy Manager RM) and anOPC library OPCL. The application APP may be e.g. a display application,a trend application or a report application. The OPC clientfunctionality OPCC is responsible for communicating with the OPC serverOPCS, using the OPC libraries OPCL. The OPC client functionality OPCCmay, utilizing data transfer protocols of lower layers (typicallyTCP/IP/Ethernet), arrange several parallel logical OPC connections withone or more OPC servers OPCS. The redundancy manager RM according to apreferred embodiment is responsible for estimating parallel logicalconnections in a manner illustrated in closer detail in FIG. 3. Theblocks illustrated in FIG. 2 may be implemented by computer program codeto be executed in one or more central processing units CPU of theworkstation WS, the computer program code being able to use aconventional operating system.

FIG. 3 illustrates a redundancy method according to a preferredembodiment, implemented by an OPC client functionality OPCC andpreferably the redundancy manager RM illustrated in FIG. 2. Preferably,the status of the servers OPCS is checked 301 when the interface of theOPC servers OPCS is to be used, e.g. when information indicated by OPCdata items is to be retrieved using an OPC data transfer interface. Step301 can also be carried out at other times, e.g. in connection withsetting up logical connections, or at predetermined intervals. Thestatus of the servers OPCS may be checked 301 separately by requesting(GetStatus) status information on the OPC servers OPCS (OPC SERVERSTATUS) or by checking the status information provided by the serversOPCS automatically. If the status information (OPCSERVERSTATE) indicatesthat the OPC server operates normally, i.e. the value isOPC_STATUS_RUNNING, the server is in proper condition. Preferably, inconnection with all other status information values, the server isinterpreted to be out of order.

If, on the basis of the checking 301, 302, not a single working serverOPCS is available, no OPC interface is available 303 for the OPC clientOPCC. In such a case, it is possible to retry to activate 303 a logicalconnection later. Otherwise, the OPC client OPCC selects for its use 304one or more servers OPCS that, on the basis of the checking, operatenormally.

In order to ensure redundancy for data transfer to be provided for oneor more applications APP, at least two parallel logical connections arearranged to transfer substantially the same data units from the samedevice DEV. Typically, but not necessarily, these parallel connectionsare arranged via different servers OPCS. Different configurationsenabling the redundancy to be arranged will be illustrated in closerdetail below in connection with FIGS. 4 to 7. When the client OPCCreceives 305 data units transferred via different connections, e.g. dataof OPC items of an OPC data transfer interface, the quality informationon the received data units carrying the same payload is checked 306.

If, on the basis of the checking and comparison 306, 307, the qualityvalues are different, a data unit whose quality information indicatesthat it is of higher quality than other data units parallel therewith isselected 308 to be provided for the application APP. The process maythus move to step 308 if a data unit is found which has a better qualityvalue than that of any other parallel data unit. If at least two dataunits exist that have the same quality values that are better than thoseof possible other data units, the process moves to step 309. If in step305 no parallel data units are received at all via a logical connection,the particular logical connection is interpreted to be defective, inwhich case data units of parallel logical connections are selected to beused 308.

According to a preferred embodiment, the quality information to becompared consists of quality information determined by the server OPCSfor quality bit fields of OPC items. Quality information can bedetermined for the OPC items on the basis of quality statuses availableat the OPC data transfer interface. Quality bits indicating qualityinformation are arranged in the following manner:QQSSSSLL.

QQ bits determine three possible quality statuses (bad, good,uncertain). SSSS bits are substatus bits whose different values enableeach quality status to be further defined. LL bits are limit bits thatmay provide diagnostic information. Data units can be arranged in orderof superiority by utilizing any order criteria predetermined for the OPCclient OPCC. All (comparing the QQSSSSLL bit combinations of parallelOPC items) or only some of the quality bits determined in the OPCspecifications for the OPC items can be used in the comparison. It isalso possible to determine new fields for determining the status of dataunits and to use them for comparing parallel data units. According to apreferred embodiment, QQ bits are used in the comparison, in which casethe predetermined order criteria from the best to the worst are‘Good’→‘Uncertain’→‘Bad’. Consequently, the data units which, accordingto the QQ bits, have ‘Good’ as the quality status, have betterproperties than the rest of the data units. If the quality statuses oftwo parallel data units are ‘Uncertain’ and ‘Bad’, the best one, i.e.the data unit having the status ‘Uncertain’, is selected to be used.According to a preferred embodiment, the comparison uses bits QQSSSS,i.e. the values of the QQSSSS bits of different OPC items are compared.In such a case, the comparison of the substatus bits SSSS furtherdefines the quality statuses of the QQ bits, the comparison preferablybeing carried out if parallel data units have the same QQ bit status. Itis also possible that the client OPCC selects for use the data units ofthose logical connections only whose quality information indicates goodquality, i.e. whose value is ‘Good’.

In step 309, the time stamps of the data units to be compared arechecked. Preferably, only the data units that have the best qualityvalues are selected for step 309, preferably only the data units thathave the OPC quality status ‘Good’. If the best quality status is ‘Bad’,it can also be used. Particularly if a data unit has a ‘Last KnownValue’ (the bit value being 000101LL) as its substatus, it may still beusable since in spite of unsuccessful communication, the last knownvalue is still available. Typically, the OPC server OPCS has added atime stamp to the OPC data unit. The time stamp may indicate e.g. thepoint of time at which a measurement value contained in the payload wasmeasured. In a data structure OPCITEMSTATE describing the status of anOPC item, a time stamp may be provided either by a device (DEV) or aserver OPCS. If the device DEV supports time stamps, it may determinethe time stamp of the OPC data unit, it may e.g. timestamp themeasurement value.

On the basis of the checking 309, a data unit having the most recenttime stamp is selected. This data unit is transferred to be used by oneor more applications APP processing the data units. The application APPis thus always provided with one data unit which is as high-quality andnew as possible. The way in which the application APP processes thereceived data unit depends on the application and is irrelevant to theinvention.

Thus, the redundancy of transfer of a necessary data unit from thedevice DEV, e.g. a setting or measurement value, can thus be implementedby utilizing the quality and/or time information on the data units. Itis also possible to transfer data from the client OPCC to the serverOPCS via parallel logical connections. If the logical connections arearranged via different servers, each server is provided with the samedata. In such a case, however, preferably only one server OPCS writesfurther to the device DEV.

According to a preferred embodiment, steps 306 to 310 illustrated inFIG. 3 are performed on each group of data units formed by parallel dataunits containing substantially the same payload. After step 310, theprocess may thus start waiting for the next data unit to be received andcompare it with parallel data units transferred via other logicalconnections. It is also possible to perform steps 306 to 310 only onsome of the received data unit groups: For example, checking isperformed on every fifth received data unit group, wherein a data unitis selected, according to step 308 or 310. Next, the next four dataunits received via the same logical connection as the data unit selectedon the basis of the checking can be automatically selected to bedelivered to the application.

The order of superiority of the data units (on the basis of qualityinformation or some other property information on the data units) can bedetermined in accordance with the configuration, which enables ordercriteria for different purposes and situations to be customized in aversatile manner. The order criteria to be used and the applicationthereof can be selected e.g. on the basis of the data to be transferred.Different applications APP could also be provided with different ordercriteria according to which the superiority of parallel data units is tobe determined.

The functionality illustrated in FIGS. 2 and 3 enables the redundancy ofdata transfer to be centralized in the redundancy manager RM of the OPCclient OPCC, and no redundancy functionality needs to be arranged in theOPC servers OPCS or applications APP (if desired, additionalredundancies can naturally also be provided between applications andservers). Consequently, a preferred embodiment of the invention does notrequire any additional functionality in the OPC servers OPCS or in thedevices DEV connected to the OPC servers.

It is to be noted that the client OPCC (redundancy manager RM) mayimplement only some of the steps illustrated in FIG. 3. For example, theserver checking and selection in steps 301 to 304 is by no meansnecessary, and no time stamps necessarily need to be used for selectingdata units (neither do all time units necessarily have usable timestamps).

FIG. 4 illustrates a redundancy configuration according to a firstembodiment, wherein redundancy is arranged for an OPC server OPCS(including connections to one or more devices DEV), an OPC clientfunctionality OPCC and an application APP. The servers OPCS provide bothOPC clients OPCC with the same data units. Preferably in the mannerillustrated in FIG. 3, the OPC clients OPCC select a server and the bestones from among the parallel data units on the basis of the propertyinformation on the parallel data units and the predetermined ordercriteria (on the basis of quality and/or time stamp information) andtransmit them to the applications APP. In the embodiment of FIG. 4,preferably one of the applications APP is active, in which case thepassive application and the data units provided thereto by the OPCC areused when problems are detected in the operation of the activeapplication. Preferably, the application used in the OPCC is such anapplication which uses a connection the data units transferred via whichconnection are the data units that, on the basis of the propertyinformation on the data units, are the best ones. The embodiment of FIG.4 enables redundancy to be arranged for the connection between theserver OPCS and the device DEV, the connection between the server OPCSand the client OPCC, redundancy managers RM and applications APP.

FIG. 5 illustrates a redundancy configuration according to a secondembodiment, wherein an application APP utilizes two OPC clientfunctionalities OPCC. Redundancy is thus arranged for the OPC serverOPCS and the OPC client functionality OPCC. The redundancy manager RMmay operate as illustrated in FIG. 3, i.e. it may always select dataunits delivered to the OPCC via a better logical connection for theapplication APP. The application APP selects, according to its owncriteria, a data unit delivered by another OPCC for its use. The APP mayselect the data unit on the basis of the properties of its payload, e.g.on the basis of the time stamp determined in the payload. In theembodiments disclosed in FIGS. 4 and 5, it must be ensured that only oneof the clients OPCC transmits data (write connection) back to theservers OPCS and further to one or more devices DEV.

FIG. 6 illustrates a redundancy configuration according to a thirdembodiment, wherein redundancy is arranged for OPC servers OPCS1, OPCS2,thus providing an OPC client OPCC with parallel logical connections. TheRM may select a data unit, preferably in the manner illustrated in steps305 to 310 in FIG. 3. FIG. 6 illustrates a situation wherein the clientdevice OPCC has a need to receive data units from three devices DEV1,DEV2 and DEV3. Three connection pairs are then formed: 1-1, 2-1; 1-2,2-2 and 1-3, 2-3. In the example of FIG. 6, it is detected, preferablyon the basis of the checking illustrated in FIG. 3, that the data unitstransferred to the device DEV1 via the connection 1-1- used by the OPCserver OPCS1 are of poor quality, or that they are missing altogether,in which case the connection 2-1 of the server OPCS2 and the data unitstransferred therethrough are used in the application APP of the OPCclient device OPCC. However, the connection 2-2 provided by the serverOPCS2 for the device DEV2 is defective, so the OPCC selects the dataunits transferred from the device DEV2 via the connection 1-2 and theserver OPCS1 to be used by the application APP. From the device DEV3,the connections 1-3 and 2-3 are in proper condition, and it is assumedthat the data units transferred via the particular connections alsootherwise remain equal in quality, which always enables the OPCC toselect the most recent data unit for the application APP. Thus,according to a preferred embodiment, the OPCC may use several differentservers for transferring data units transferred from different devices,whereby it is always possible to arrange data transfer via a parallellogical connection when a logical connection (or a server) fails.

FIG. 7 illustrates a redundancy configuration according to a fourthembodiment, wherein two parallel logical connections are provided via aserver OPCS for transferring data of a device DEV (one or more). Thisenables redundancy to be arranged particularly for the connectionbetween the server OPCS and the device DEV as well as for the connectionbetween the server OPCS and a client OPCC.

It is to be noted that in addition to the configurations shown in FIGS.4 to 7, several other configurations may also exist wherein the presentredundancy method can be utilized. In the embodiment of FIG. 7, forexample, wherein the parallel logical connections are arranged via oneserver OPCS, two applications APP instead of one application APP may beprovided, as illustrated in FIG. 4.

The redundancy method illustrated above may be utilized at any OPCinterface the data units according to which comprise quality and/or timeinformation. A server OPCS providing an OPC data transfer interface (OPCDataAccess) comprises several objects: server, group and item. An OPCserver object maintains information on the server and stores OPC groupobjects. Through adding, removing and modifying the OPC group objects, aclient OPCC is able to organize information according to the needs ofapplications APP. An OPC group object maintains information on itselfand provides means for adding, removing and organizing OPC items. Agroup may e.g. present a certain report which an operator monitoring aprocess desires to get about a part of the process. A group thusdetermines several OPC items that in a group identify the data sourcefrom which the data is obtained. The information indicated by the OPCitem may be e.g. a process value or a set value. The OPC client OPCC maydetermine e.g. a certain time interval during which information isretrieved from the data sources indicated by the OPC items determined bythe group. If necessary, the OPCS retrieves information typically viaproprietary interfaces from one or more devices DEV (a softwareinterface and/or a hardware interface) and organizes the informationinto a format determined by an OPC data transfer interfacespecification. The information indicated by the OPC item can then betransmitted to the client OPCC. This can be called OPC item transfer toOPC client OPCC, the OPC data unit to be transferred providing theclient with a value (data contained in the address), quality, time stampand a data type. It is also possible for the OPC client OPCC to write tothe address indicated by the OPC item. Data units provided by the OPCalarm and event interface (OPC Alarm&Event) or the OPC historical datainterface (OPC HistoricalData) may also comprise quality informationand/or time stamps that can be utilized in redundancy as describedabove.

It is obvious to one skilled in the art that as technology advances, thebasic idea of the invention can be implemented in many different ways.The invention and its embodiments are thus not restricted to theabovedescribed examples but may vary within the scope of the claims. Inaddition to the OPC standard interfaces described above, the inventioncan also be applied in connection with other interfaces, e.g. in aprotocol called DAIS (Data Acquisition from Industrial Systems) of anOMG (Object Management Group) protocol.

The invention claimed is:
 1. A method for arranging redundancy in aprocess control system comprising at least one client device, at leastone data source and at least one server device providing data transferbetween the at least one client device and the at least one data source,the method comprising: arranging, for one client device of the at leastone client device, at least two parallel logical connections fortransferring the same data in parallel between one data source of the atleast one data source and the one client device, checking, at the oneclient device, property information on parallel data units transferredby the parallel logical connections, the one client device beingprovided with predetermined order criteria for the property information,comparing, at the one client device, property information on theparallel data units transferred by the parallel logical connections, andselecting data units, based on the comparing step, delivered via theparallel logical connections, having, according to the predeterminedorder criteria, the best property information for the use of one or moreapplications processing the data units, wherein the parallel logicalconnections between the one client device and one server device of theat least one server device are arranged according to an OPC standard{OLE (Object Linking and Embedding) for Process Control}, and for eachcomparing step, the selecting step selects a single data unit, havingthe best property information, from among the parallel data units, anddisregards other data units from among the parallel data units.
 2. Amethod as claimed in claim 1, wherein the property information to becompared includes quality information determined by the one serverdevice for quality bit fields of OPC items, and the OPC items of theparallel logical connections whose quality information indicates thebest quality are used at the one client device.
 3. A method as claimedin claim 2, wherein the OPC items are selected on the basis of qualitystatuses in the following order of superiority, from the best quality tothe worst quality: ‘Good’ ® ‘Uncertain’ ® ‘Bad’.
 4. A method as claimedin claim 1, wherein the property information to be compared includestime stamps of the parallel data units, and the parallel data units ofthe parallel logical connections whose time stamps are the most recentones are selected at the one client device.
 5. A method as claimed inclaim 4, wherein the quality information on the parallel data units isfirst compared, and the time stamps are compared in response to thequality information indicating that different data units are equal inquality.
 6. A method as claimed in claim 1, wherein at least twoparallel applications are arranged in the one client device to implementthe parallel logical connections, and such an application which uses aconnection the parallel data units transferred via which connection are,on the basis of the property information on the parallel data units, thebest ones, is used at the one client device.
 7. A method as claimed inclaim 1, wherein at least two parallel client interfaces are arranged inthe one client device to implement the parallel logical connections, andthe data carried by the client interface the parallel data unitstransferred via which are, on the basis of their property information,the best ones, is selected.
 8. A method as claimed in claim 1, whereinthe parallel logical connections are arranged via different serverdevices, thereby the server statuses of the different server devices arechecked before comparing the property information on the parallel dataunits, and the property information is compared only on the paralleldata units transferred via the server units whose status indicates thatthe server units operate normally.
 9. The method as claimed in claim 1,wherein the one client device is a single device.
 10. A process controlsystem comprising at least one client device, at least one data sourceand at least one server device providing data transfer between the atleast one client device and the at least one data source, wherein atleast two parallel logical connections can be arranged for one clientdevice of the at least one client device for transferring the same datain parallel from one data source of the at least one data source, theone client device is configured to check property information onparallel data units transferred by the parallel logical connections, theone client device being provided with predetermined order criteria forthe property information, the one client device is configured to compareproperty information on the parallel data units transferred by theparallel logical connections to obtain comparison results, the oneclient device is configured to select data units, based on thecomparison results, delivered via the parallel logical connections,having, according to the predetermined order criteria, the best propertyinformation for the use of one or more applications processing the dataunits, the parallel logical connections between the one client deviceand one server device of the at least one server device are arrangedaccording to an OPC standard {OLE (Object Linking and Embedding) forProcess Control}, and for each comparing, the one client device selectsa single data unit, having the best property information, from among theparallel data units, and disregards other data units from among theparallel data units.
 11. The process control system as claimed in claim10, wherein the one client device is a single device.
 12. A device of aprocess control system, comprising a client device functionality forcommunicating with at least one server device providing data transfer inthe process control system between the device and at least one datasource, wherein the device is configured to set up at least two parallellogical connections for transferring the same data in parallel from onedata source of the at least one data source, the device is configured tocheck property information on parallel data units received by theparallel logical connections, the device being provided withpredetermined order criteria for the property information, the device isconfigured to compare property information on the parallel data unitstransferred by the parallel logical connections to obtain comparisonresults, the device is configured to select data units, based on thecomparison results, delivered via the parallel logical connections,having, according to the predetermined order criteria, the best propertyinformation for the use of one or more applications processing the dataunits, and the parallel logical connections between the device and oneserver device of the at least one server device are arranged accordingto an OPC standard {OLE (Object Linking and Embedding) for ProcessControl}, and for each comparing, the device selects a single data unit,having the best property information, from among the parallel dataunits, and disregards other data units from among the parallel dataunits.
 13. A device as claimed in claim 12, wherein the propertyinformation compared by the device includes quality informationdetermined by the one server device for quality bit fields of OPC items.14. A device as claimed in claim 12, wherein the property informationcompared by the device includes time stamps of the parallel data units.15. The device as claimed in claim 12, wherein the device is a singleclient device.
 16. A non-transitory computer readable medium storing acomputer program for controlling at least one device of a processcontrol system, wherein the computer program controls the at least onedevice to: arrange at least two parallel logical connections fortransferring the same data in parallel between one data source of atleast one data source and one device of the at least one device via oneserver device of at least one server device, check property informationon parallel data units transferred by the parallel logical connections,compare property information on the parallel data units transferred bythe parallel logical connections to obtain comparison results, selectthe data units, based on the comparison results, having, according topredetermined order criteria, the best property information for the useof one or more applications processing the parallel data units, whereinthe parallel logical connections between the one device and the oneserver device are arranged according to an OPC standard {(OLE (ObjectLinking and Embedding) for Process Control}, and for each compare step,the select step selects a single data unit, having the best propertyinformation, from among the parallel data units, and disregards otherdata units from among the parallel data units.
 17. The non-transitorycomputer readable medium as claimed in claim 16, wherein the propertyinformation to be compared includes quality information determined bythe one server device for quality bit fields of OPC items, and the OPCitems of the parallel logical connections whose quality informationindicates the best quality are used at the one client device.
 18. Thenon-transitory computer readable medium as claimed in claim 17, whereinthe OPC items are selected on the basis of quality statuses in thefollowing order of superiority, from the best quality to the worstquality: ‘Good’ ® ‘Uncertain’ ® ‘Bad’.
 19. The non-transitory computerreadable medium as claimed in claim 16, wherein the one device is asingle client device.